Angular accelerometer

ABSTRACT

An angular accelerometer which employs an annular conduit filled with a dielectric fluid. The conduit is blocked at at least one point and there are piezoresponsive elements mounted so as to have the fluid directly apply pressure thereto. The elements are mounted adjacent to the blocked portion of the conduit, one on either side thereof. The structure is adapted for use with a drilling bit in order to indicate eccentricities during drilling as well as to measure instantaneous angular acceleration.

Feb. 5, 1974 3,293,921 12/1966 Riordan 73/516 LM 3,303,893 2/1967 Vameyet a1. 175/26 3,374,341 3/1968 73/151 X n Rm mm mh on ma L m NWT m F m mme A m m H w n.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Oct. 19, 1971 Primary Examiner-Marvin A. Champion AssistantExaminerRichard E. Favreau Attorney, Agent, or Firm-K. E. Kavanagh eta1.

[21] Appl. No.: 190,639

Related US. Application Data [62] Division of Ser. No. 859,768, Sept.22, 1969, Pat.

ABSTRACT An angular accelerometer which employs an annular conduitfilled with a dielectric fluid. The conduit is 52 us. 175/40, 73/516 LMblwked at at least one Point and there are v 51 1m. (:1. G01p 15/00Sponsive elements mounted as have the fluid 58 175/40, 41, 26; 73/516,488,

rectly apply pressure thereto. The elements are Field of Search...

73/494; 166/66 mounted adjacent to the blocked portion of the conduit,one on either side thereof. The structure is adapted for use with adrilling bit in order to indicate [5 6] References Cited UNITED STATESPATENTS eccentricities during drilling as well as to measureinstantaneous angular acceleration.

2 963,099 12/1960 Gianelloni......................... 175/26 X 3,225,83012/1965 Livingston 175/45 X 3 Claims, 5 Drawing Figures PATENTED 51974snmi 0F 2 Axis f weds/5 ANGULAR ACCELEROMETER This is a division of US.Pat. application Ser. No. 859,768, filed Sept. 22, I969, now US. Pat.No. 3,693,460 issued Sept. 26, I972.

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This inventionconcerns angular accelerometers, in general, and particularly relates toan improved accelerometer that employs an annular fluid filled conduitwith piezoelectric transducers subject to fluid pressures generatedthereby. In addition, the invention concerns a combination of such anangular accelerometer mounted on or adjacent to a drill bit.

2. DESCRIPTION OF THE PRIOR ART Angular accelerometers having fluidfilled circular conduits are well-known insofar as that much structuregoes. However, the arrangements employed provided for various types ofsensing elements that all have had limitations as to the sensitivity.Also, some of the arrangements employed electrically conductive fluidsand they made use of such property of the fluid to measure fluid flow indetermining accelerations. These various structures also had otherdrawbacks, e.g. such arrangements have had mechanically moving elementswhich are subject to damage under rough handling and operatingconditions, or they were lacking in adequate sensitivity, or both.

SUMMARY OF THE INVENTION Briefly, the invention concerns an angularaccelerometer which comprises in combination an annular conduit filledwith a dense dielectric fluid. It also comprises at least one barrier insaid conduit which provides at least two ends for said conduit, and apair of piezoelectric transducers one adjacent to each of said conduitends. Finally, it also comprises means for mounting said transducerssubject to pressure changes in said fluid.

Also, the invention may be briefly described as an angular accelerometerfor measuring the eccentricity of rotation of a rotating member whichhas an axis of rotation. This accelerometer comprises an annular conduithaving an axis and being filled with a dense dielectric fluid, and meansfor mounting said conduit with the axis thereof coincidingwith said axisof rotation. It also comprises at least two barriers diametricallyopposed in said conduit providing two ends each for said conduit, and apair of piezoelectric transducers adjacent to each of said barriers.Finally, it comprises means for mounting said transducers subject topressure changes in said fluid.

Once more, briefly, the invention concerns a combination with a drillbit for drilling oil wells or the like. The combination includes acollar adjacent to said bit. The combination also includes an angularaccelerometer having an axis of rotation, and means for mounting saidaccelerometer with said axis of rotation coinciding with thelongitudinal axis of said bit.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects andbenefits of the invention will be more fully set forth below inconnection with the best mode contemplated by the inventor of carryingout the invention, and in connection with which there are illustrationsprovided in the drawings wherein:

FIG. 1 is a perspective showing an accelerometer structure according tothe invention;

FIG. 2 is a schematic illustration indicating a modified accelerometerfor use to indicate eccentricities;

FIG. 3 is another schematic illustration like FIG. 2 but showing amodification for obtaining additional signals to provide eccentricitymeasurements;

FIG. 4 is a schematic circuit diagram illustrating the way in whichoutput signals from an accelerometer according to FIG. 3 may be treated;and

FIG. 5 is a schematic showing, in longitudinal elevation, with a portionin longitudinal cross section illustrating the combination of a borehole drilling bit and an accelerometer all according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1

FIG. 1 illustrates an accelerometer which was constructed fordetermining operating characteristics thereof. There is a supportingframe 11 having an arcuate scale 12 attached to one side by a bracket13. The body of the accelerometer itself is constructed with upper andlower flat ring-like annular portions 16 and 17. These rings aresupported on a disk 18 which in turn has a central hub 19 attachedthereto.

There are vertical supporting torsion springs 22 and 23, that areattached to the hub 19 at the center or axis of it and of the disk 18.They extend vertically up and down to upper and lower hubs 26 and 27respectively. These are mounted on the frame 11 at the center of anupper panel 28 and a lower panel 29 respectively.

The torsion springs 22 and 23 may take any feasible form. For example,they might be made up of a flat ribbon of spring metal or otherresilient material which will act to provide the required torsionalproperties. This could, of course, be a single ribbon that passesthrough the hub 19. It would be fastened to the hub in any feasiblemanner (not shown) so as to support the body of the accelerometer.

In between the upper and lower body rings 16 and 17, there is a channelor conduit 32 that is filled with a dense dielectric fluid, e.g. oilhaving appropriate flow characteristics. The conduit is rectangular incross section. It is formed by providing a pair of spacer rings 33 and34. These spacers are situated in between the body rings 16 and 17 andextend radially a distance that is approximately one-third of the radialwidth thereof. The inner spacer ring 33 is flush with the inside radiusof the body rings 16 and 17. Similarly, the outer ring 34 extends withits maximum radius located flush with the outside radial walls of bodyrings 16 and 17.

These rings 33 and 34 are mounted in fluid tight contact with the upperand lower elements 16 and 17 so as to form the channel or conduit 32therebetween. However, it will be observed that the conduit 32 does notextend all the way around the annulus of the accelerometer. There is abarrier that is created by a solid piece 37 in the conduit 32. This isfitted into and completely fills up the conduit 32 so as to provide twoends of the conduit. There is located at each of these two ends one of apair of transducers 38 and 39. The transducers are embedded in the ringor body member 16, with the lower (as viewed in FIG. 1) face thereof indirect contact with the fluid in the conduit 32. Consequently pressurecreated by the fluid in conduit 32 is directly applied to a face of eachof the transducers 38 and 39 at all times.

The transducers 38 and 39 are constructed with any feasibleconfiguration. However, they are preferably cylindrical in shape withthe diameter equal to the radial width of the channel or conduit 32. Inthis manner the fluid pressure, caused by the fluid in conduit 32 willbe applied over the entire bottom face (as viewed in FIG. 1) of bothtransducers 38 and 39. The upper face of the transducers are in directcontact with (and thus are constrained by) the material of the upperbody ring 16. Consequently, fluid pressures from the conduit 32 will actto compress vertically the transducers 38 and 39.

These transducers 38 and 39 might be constructed of piezoresistivematerial and appropriate electric circuitry would be provided. It willbe understood that piezoelectric tranducers generate low level e m jswhile the use of piezoresistive elements would require eitherconstant-current or constant-voltage exitation. However, in theillustrated structure the transducers are piezoelectric elements.

There is appropriate electrode structure for each of the transducers asis schematically indicated at the top of transducer 38 in FIG. 1. Suchelectrodes act to carry the signals, in the form of electricalpotentials, out from the transducer elements to appropriate amplifiercircuits (not shown) over appropriate insulated conductors, as indicatedby the illustration of a twisted pair 42.

There is an index pointer 43 located on the upper body ring 16 of theaccelerometer. This points towards the scale 12 so as to indicaterotational positions of the body of the accelerometer relative to itssupporting frame 11. It will be appreciated that the accelerometerillustrated is a laboratory type of instrument and the readings whichare obtained will be generally sine wave in form. Such signals willindicate the amplitude of the acceleration that takes place as the bodyof the accelerometer swings back and forth from its neutral positionunder the influence of the torsion springs 22 and 23. It has beendiscovered that an accelerometer according to this invention providesunexpectedly high sensitivity, in indicating acceleration forces.

FIGS. 2-4

It is to be noted that the invention is applicable to a particular usefor an angular accelerometer, i.e., to indicate eccentricity ofrotation. In order to provide such result, it will be appreciated thatthe accelerometer structure must be mounted with its physical axiscoinciding with the axis of rotation of the body that is to have theeccentricity of its rotation measured.

Referring to FIG. 2 there is shown, schematically, an accelerometer 50which is constructed in accordance with the same principles as theaccelerometer illustrated in FIG. 1 but with an additional barrier inthe fluid conduit or channel. These barriers are located diametricallyopposite to one another, as indicated. Thus, there are two piezoelectrictransducers 51 and 52 (schematically illustrated as rectangles withhorizontal closely spaced lines) which have a barrier 55 therebetween.Diametrically opposite those two transducers and barrier, there isanother pair of transducers 58 and 59, with a barrier 60 therebetween.

It will be appreciated that because of the peripherally disposed liquidmasses and the symmetrical arrangement of the multiple transducers, theinstrument is capable of sensing eccentric rotation. Thus, even if thereis constant velocity rotation, signals will be obtained from both pairsof transducers if it is eccentric rotation. Furthermore, by having thetransducer pairs located diametrically opposite one another, there is anaxis of symmetry and readings may be related to components ofeccentricity with respect to the geometrical center or axis. This isindicated by the captions and the symbols which show displacement of thecenter of rotation from a point 63 that is the geometrical axis, to apoint 64 that is an instantaneous center of rotation. The latter has adisplacement that is indicated by the arrow which representseccentricity of the rotation at a given moment.

FIG. 3 is a schematic showing similar to that of FIG. 2. However, itillustrates an accelerometer that is especially designed for providingeccentricity readings. It has the fluid conduit divided into four equalsections with pairs of transducers at the conduit ends that are locatedat diametrically opposite barriers. Such opposite barrier pairs areoriented at right angles to one another, and the barrier locations areindicated by the captions E E E and E.,. This arrangement issubstantially the same in principle as that of the FIG. 2 showing andthe points which were explained above are provided with the samereference numbers but with a prime mark added to each.

It will be appreciated that the same quadrature relationship might beobtained by mounting two accelerometers like that of FIG. 2 with thegeometric axes coinciding but with the fluid conduits parallel and withthe barriers located at right angles.

It will be understood that in all modifications of the accelerometerthere may be a restricted passage (not shown) which crosses the barrierin each case. This would be provided in order to permit slow seepage ofthe fluid from one side of the barrier to the other so as to providelongterm pressure equalization. The size of such passage would be suchthat it would not affect sensitivity or speed of response.

In FIG. 4 there is shown a block diagram form of the electrical circuitfor handling the signals from an arrangement like that illustrated inFIG. 3. Thus, there are four amplifiers 67, 68, 69 and 70. These receivethe signals from the transducer pairs that are related to each of thebarrier locations E E E and E respectively. In this manner, the signalsfrom diametrically opposed pairs of transducers at barrier locations Eand E may be subtracted in a difference or subtraction circuit 73.Thereafter this signal is squared by an appropriate circuit element 74,the output of which leads to a summation circuit 75.

Similarly, the signals from the transducer pair locations E and E (whichare also diametrically opposed to one another but are oriented at rightangles to the other locations) are subtracted in a circuit element 78and squared in element 79 followed by application to another input ofthe summation circuit 75.

The output of the summation circuit has the square root of the sum takenby a circuit element 80. The output of 80 feeds into an amplifier 81prior to application of the signal to a recorder for recordingeccentricity signals. It will be understood that the net differencesbetween the signals of opposed pairs of elements will be proportional tothe respective normal components of the eccentricity. And, consequently,the square root of the sum of the squares of signal differences will beproportional to the total instantaneous eccentricity of the center ofrotation.

The arrangement according to FIGS. 3 and 4 also 9 provides for theangular acceleration signal measure- FIG. 5

FIG. 5 illustrates application of an accelerometer according to theinvention to a bore hole drill bit 90, which is schematicallyillustrated by the dotted lines shown. The bit is carried by a collar 91and attached thereto is a short section of pipe 92 that is threadedlyattached to the collar 91 as schematically indicated. Pipe 92 also has athreaded joint 95 at the other end thereof. This is for use in attachingthe unit to the lower end of a string of drill stem (not shown).

Within the section 92 there is a flexible material liner 96 that formsthe walls of an internal passage 97 which connects the centralpassageway of the drill stem to the collar 91 and bit 90 of the drill.This permits passage of the drilling mud or other fluid that is employedduring drilling operations.

Located near the lower end (as viewed in FIG. 5) of the pipe section 92there is an accelerometer 99, that is schematically indicated in FIG. 5.This will take the form of an accelerometer according to the foregoingdescriptions. Preferably, it will include the elements needed todetermine both eccentricity and acceleration during the rotation of thedrilling operation.

Below the accelerometer 99, as viewed in FIG. 5, there is a support ring100. Similarly,just above the accelerometer (as viewed in FIG. 5) thereis another support ring 101. These rings act to hold the accelerometer99 in place within the pipe section 92.

Near the other end of the pipe section 92 there is another ring 102 thatsupports the upper end of the liner 96 and forms a seal to keep wellfluids out of an annular space 105 that is located between the ring 102and the ring 101. In this space 105 there will be located the elements(not shown) for handling the electrical signals. This includes arecorder or recorders to make a permanent record of the acceleration andeccentricity conditions. It will be observed that the structure providesfor a compact and rugged yet sensitive instrument that may record theforces of acceleration and concurrently the amount of eccentricityencountered during drilling operations.

The operation of an accelerometer according to the invention, both as toangular acceleration and eccentricity measurements, may be analyzedmathematically as follows:

Piezoelectric Element Response The magnitude and polarity of inducedpiezoelectric surface charges are proportional to the magnitude anddirection of the applied force. The charge Q is given by where d is thepiezoelectric constant and F is the force applied.

For an element of face area A and thickness t of a material with Youngsmodulus Y, the force F may be expressed by the equation F AY (At)/twhere At is change in thickness.

Therefore, the charge Q becomes Q d (AY (At)/t) and since the voltage Eresulting from the electrode charge Q is given by where C is thecapacitance between the electrodes and is equal to e A/t where e is thedielectric constant, the emf E becomes E Qt/eA E dY (At)/e Substitutingfor the Youngs modulus Y," the voltage becomes E (dt/eA) F and in termsof pressure where pressure p F /A E l/ P Defining the ratio d/e as thevoltage coefficient g E=gtp.

Thus, the voltage appearing across the faces of the piezoelectricelement is equal to the product of the piezoelectric voltage coefficientof the element material, the thickness separating the faces, and thepressure applied.

Accelerometer Motion and Pressure Since the force applied by the liquidcolumn in the torus against the face of the piezoelectric element iswhere 8 is the liquid density and V is the liquid volume, then thepressure p is given by p=K,7a

K, being a constant accounting for the liquid density and dimensions ofthe device.

The last equation (2) above states that the instantaneous unit pressureat the face of the piezoelectric element is proportional to the productof the radius about the instantaneous center of rotation and the angularacceleration.

Pressure and Induced EMF Equation (1) above gives the induced EMF as EFor piezoelectric elements of given coefficients and thickness, theproduct gt may be replaced by the constant K Then E=K P Equation (2)above gives the pressure as p K, F 11 Combining the last two equations('2) and (3) E K, K 7 a Equation (4) states that for given geometry andpiezoelectric elements of predetermined characteristics, the induced emfsignal is uniquely determined by the product of the instantaneous radiusand the angular acceleration. This fact forms the basis for theoperation of the subject angular accelerometer cum eccentricitydetector.

While the invention has been described above in considerable detail inaccordance with the applicable statutes, this is not to be taken as inany way limiting the invention but merely as being descriptive thereof.

I claim:

1. In combination with a drill bit, having a longitudinal axis and beingadapted for drilling oil wells or the 8 like,

a collar adjacent to said bit, an angular accelerometer for measuringeccentricity of rotation of a rotating member having an axis ofrotation, said accelerometer comprising an annular conduit having anaxis and being filled with a dense dielectric fluid, means for mountingsaid conduit with the axis thereof coinciding with said axis ofrotation, at least two barriers diametrically opposed in said conduitproviding two ends each for said conduit, a pair of piezoelectrictransducers adjacent to, but on opposite sides of each of said barriers,means for mounting said transducers subject to pressure changes in saidfluid due to acceleration forces, electrical circuit means foramplifying and combining the output signals from said pairs oftransducers in order to provide measurement of the eccentricity ofrotation of said rotating member, and means for mounting saidaccelerator with said axis of rotation coinciding with the longitudinalaxis of said bit. 2. The combination according to claim 1 wherein saidmounting means is integral with said collar.

3. The combination according to claim 2 further including an annularchamber in said collar for housing said accelerometer and relatedelements, and passage means axially through said collar for allowingdrilling fluid to flow therethrough.

1. In combination with a drill bit, having a longitudinal axis and beingadapted for drilling oil wells or the like, a collar adjacent to saidbit, an angular accelerometer for measuring eccentricity of rotation ofa rotating member having an axis of rotation, said accelerometercomprising an annular conduit having an axis and being filled with adense dielectric fluid, means for mounting said conduit with the axisthereof coinciding with said axis of rotation, at least two barriersdiametrically opposed in said conduit providing two ends each for saidconduit, a pair of piezoelectric transducers adjacent to, but onopposite sides of each of said barriers, means for mounting saidtransducers subject to pressure changes in said fluid due toacceleration forces, electrical circuit means for amplifying andcombining the output signals from said pairs of transducers in order toprovide measurement of the eccentricity of rotation of said rotatingmember, and means for mounting said accelerator with said axis ofrotation coinciding with the longitudinal axis of said bit.
 2. Thecombination according to claim 1 wherein said mounting means is integralwith said collar.
 3. The combination according to claim 2 furtherincluding an annular chamber in said collar for housing saidaccelerometer and related elements, and passage means axially throughsaid collar for allowing drilling fluid to flow therethrough.